The practice of using a liquid impregnant such as sodium silicate or an unsaturated polyester or an aerophobic liquid impregnant for the purpose of infallibly sealing invisible pinholes in die-cast products and cast products of aluminum, zinc, and other metals; completely filling and sealing pores in sintered metal parts; preventing plated products from blowing by subjecting the parts to the treatment of impregnation before the step of plating; and sealing minute pores in porous non-metallic parts such as wood and ceramics has been in vogue.
For example, the impregnation of a given part is attained by: degreasing and cleaning the part; subjecting the cleaned part to vacuum aspiration in a vacuum tank, thereby removing entrapped air from the minute pores in the part; immersing the part in a bath of an organic liquid impregnant such as an aerophobic resin; maintaining the part in a vacuum; and subsequently exposing the immersed part to the atmospheric pressure, thereby causing the liquid impregnant to permeate the minute pores. In this case, the impregnation can be enhanced by supplying compressed air to the site of impregnation. Then, the liquid impregnant is returned to the reservoir and the part which has undergone the impregnation is centrifuged to expel the liquid impregnant adhering to the surface of the part. Thereafter, the part is cleaned with a detergent to remove the liquid impregnant still remaining on the surface of the part or in the tapped holes. The treatment of impregnation is finished by subjecting the part to a treatment for curing.
In this treatment of impregnation, the effluent to be discharged from the site of treatment is the spent detergent which results when the liquid impregnant, remaining only slightly on the surface of the part which has undergone the treatment of impregnation, is washed off with a detergent. Heretofore, organic solvents such as trichloroethane and fluorinated hydrocarbon have been used as detergents. On account of the anxiety about the environment and the concern about cost, the use of such organic detergents has been giving place to the use of water for the cleaning. In consequence of this trend, there has arisen the necessity for separating and recovering the liquid impregnant contained in the effluent before the effluent is discarded from the plant. This separation of the liquid impregnant from the effluent is a difficult matter.
For the treatment of the effluent of this nature, the following methods have been available to date.
(1) Method of adsorption: The liquid impregnant which is an organic substance is removed by adsorption using an adsorbent such as activated carbon. By this treatment, both the BOD and COD contents in the effluent can be lowered to below several ppm. Thus, this method may well be called a method for final treatment productive of an effluent tolerable for release into rivers. (2) Method of microorganic decomposition: This method, represented by the version resorting to the action of activated sludge, allows the same degree of treatment as the method of adsorption by the decomposition of organic matter with aerobic bacteria. (3) Method of combustion: The effluent is dispersed in heavy oil and the resultant mixture is burnt by spraying into a flame. Since this combustion produces carbon dioxide gas and water, this method effects perfect detoxification of the effluent. (4) Method of thermal polymerization: This method utilizes the phenomenon that an impregnant polymerized in water is insoluble in water. Generally, the water containing the impregnant is heated for the purpose of promoting the reaction. The treated effluent is subjected to solid-liquid separation by filtration or settling (difference in specific gravity). The treated effluent has a COD content of from some thousand to some tens of thousand ppm and, therefore, is generally not allowed to be released into rivers. It is, either subjected to another final treatment or reclaimed as washing water. (5) Method of separation by flocculation: This method, primarily intended for the removal of suspended matter from water, is capable of causing incorporation of a water-soluble liquid impregnant in the product of flocculation. The treated effluent is substantially equal in quality to that obtained by the method of thermal polymerization.
The methods of (1) and (2) are both classical ways of waste water disposal and may well be called common techniques. Their execution, however, entails a space and cost for installation of apparatus for impregnation. The exclusive use of these methods as means for impregnation is not practically feasible, with the running cost as a contributory factor. The method of combustion of (3) operates with a relatively simple apparatus and relies on treatment simply on the phenomenon of combustion, and may well be called a method of popular use. The largest problem confronting this method is the fact that the vaporization of water, which accounts for most (about 90%) of the effluent, consumes a large volume of energy, and the expense of fuel for the vaporization brings a huge addition to the running cost. The method of (4) utilizes the reactivity of the impregnant and enjoys popular acceptance next to the method of (3). This method, however, necessitates numerous auxiliary devices such as devices for injection of a flocculating auxiliary and a pH adjusting agent, a reaction tank, a separation tank, and a device for cooling the treated effluent, for example. Moreover, the by-product of the reaction defies elucidation and control. These factors, coupled with the cost of energy for heating and cooling, prevent this method from being disseminated. The method of flocculation of (5) necessitates the same apparatus as the method of (4), except it does not use the devices used in method (4) for heating and cooling. Since the floc produced by this method has low strength, the liquid impregnant contained in the floc redisperses when the speed of filtration for separation is heightened, and the load exerted on the device for filtration increases when the speed of filtration is slowed.
The conventional methods of treatment, except for the method of thermal polymerization utilizing the reactivity of the liquid impregnant, turn out to be those generally employed for the disposal of other industrial effluents. Since the polymerization hinges heavily on temperature, the method of thermal polymerization calls for a device for temperature control and a device for cooling the treated effluent for reclamation. The other conventional methods are not satisfactory measures for the treatment of the effluent emanating from the apparatus of impregnation for the reasons given above. Thus, the desirability of developing a method of an entirely novel concept which provides a highly efficient and economically advantageous treatment of the effluent under discussion has been finding enthusiastic recognition.
An object of this invention is to provide a method for the treatment of the effluent resulting from the process of detergency, the method being affected by temperature only to a small extent, the method requiring no addition of a chemical agent, and the method producing a treated water which is not affected by a change in the effluent concentration.